In recent years, as one of drive systems for a liquid crystal display device that displays a color image, the development of a field-sequential system has been actively advanced. A typical field-sequential system is a system in which a one-screen display period (one frame period) is divided into three subfields, and each of LEDs (Light Emitting Diodes) of red (R), green (G), and blue (B) to be a light source of backlight is sequentially switched, and in synchronization therewith, an image signal of a color corresponding to the color of light of each LED is provided to a liquid crystal panel to control its transmission state, thereby performing additive color mixing on retinas of observer's eyes. According to the field-sequential system, color display can be performed without forming a plurality of sub-pixels for one pixel, thereby enabling high resolution. Further, direct use of the light from the LED eliminates the need for forming a color filter with high absorption at each pixel, thereby improving the light usage efficiency of each LED.
Meanwhile, in the field-sequential system, the lighting time for each of the LEDs of R, G, B is not the same, which causes a problem of occurrence of “color breakup” at the time of display of a moving image on a liquid crystal panel. As one of methods for solving this problem, Japanese Patent Application Laid-Open No. 2002-229531 describes a method in which one frame period is divided into not only three subfields consisting of the respective single colors of R, G, B, but a total of four subfields obtained by adding one subfield made up of a mixed color of these three colors, to make a ratio of the single color subfields small. In this case, if another color is mixed into the added subfield, the color balance of the image is lost, which makes an accurate color unreproducible, and hence the added subfield is preferably made up of an achromatic color such as black, white, or gray. Accordingly, in the following description, the subfields made up of the respective single colors of R, G, B are referred to as “R field”, “G field”, and “B field”, respectively, and the subfield made up of the achromatic color is referred to as “W field”.
FIG. 16 is a diagram showing light emission states of the LEDs, and four subfields constituting one frame, in the case of displaying an image by the field-sequential system provided with the four subfields in a conventional example. A high part of a waveform of each LED shown in FIG. 16 represents a “lighted state”, and a low part thereof represents a “non-lighted state”. In the W field, all of the red LED, the green LED, and the blue LED are in the lighted states throughout one subfield period. In the R field, only the red LED is in the lighted state throughout one subfield period. In the G field, only the green LED is in the lighted state throughout one subfield period. In the B field, only the blue LED is in the lighted state throughout one subfield period. In synchronization with the lighted state of each LED, image data of a color corresponding to the color of light of each LED is sequentially provided to the liquid crystal panel, to control a light transmission state in each of the W field, R field, G field, and B field.
FIG. 17 is a view showing an image that is viewed by the observer when the image is displayed by the field-sequential system provided with the four subfields, and FIG. 18 is a view showing an image of each subfield when the image shown in FIG. 17 is displayed by the field-sequential system. In the image shown in FIG. 17, characters of “No. 123” are displayed in blue, characters of “Please come in” are displayed in pink, characters of “This week's consultation” and consultation dates and times are displayed in green, and a consultation timetable is displayed in yellow.
When the image shown in FIG. 17 is to be displayed by the field-sequential system, as shown in FIG. 18, first, in the W field, based on brightness information obtained from an input image signal, a transmittance of the liquid crystal panel is controlled such that an area where no image of the characters, the consultation timetable, or the like is displayed in any of the subfields of R, G, B, described later, becomes a transparent display area where red light, green light, and blue light are transmitted, and an area where the images of the characters, the consultation timetable, and the like are displayed becomes a black display area where the above light cannot be transmitted, and each of the red, green and blue LEDs is simultaneously lighted. Accordingly, in the W field, the area where no image of the characters, the consultation timetable, or the like is displayed in each of the subfields of R, G, B becomes the transparent display area, and the area where the images of the characters, the consultation timetable, and the like are displayed becomes the black display area.
Next, based on color information obtained from the input image signal, the images are displayed in the order of the subfields of R, G, B. More specifically, in the R field, the characters of “Please come in” become transparent, and the red LED is lighted, thus displaying the characters in red. In the G field, the characters of “This week's consultation”, the characters representing the consultation dates and times, and the consultation timetable become transparent, and the green LED is lighted, thus displaying these characters and the consultation timetable in green. In the B field, the characters of “No. 123”, the characters of “Please come in”, and the consultation timetable become transparent, and the blue LED is lighted, thus displaying these characters and the consultation timetable in blue. Further, in each subfield, when the characters and the consultation timetable become the transparent display area, an area therearound becomes the black display area so as to prevent transmission of any of the red light, green light, and blue light.
As a result, the observer sees the characters of “No. 123” in blue, the characters of “Please come in” in pink, the characters of “This week's consultation” and the characters representing the consultation dates and times in green, and the consultation timetable in yellow. Further, the area around the characters and the consultation timetable becomes transparent, and the background of the liquid crystal display device is viewed overlapping with the images of the characters and the like.